In known mobile material distributing apparatus carried on a vehicle, a discharge pump is driven by the engine of the vehicle through a hydrostatic transmission that permits the selection of a drive ratio which produces and maintains a uniform application rate (e.g., gallons of material per square yard of road surface) through a given length of spray bar despite variations in the speed of the vehicle. In practice, the proper drive ratio of the hydrostatic transmission linking the vehicle engine and the pump is determined by making a trial run (i.e., material is circulated through the discharge pump but not discharged) and manually adjusting the drive ratio until the desired flow rate of material through the discharge pump is obtained for a given speed. Once the drive ratio is set, the rate of material flow from the discharge pump varies in accordance with the vehicle speed, thereby maintaining a uniform application rate of the material.
Often, distribution of material along the entire length of the spray bar is not desirable since the pattern is too wide for the area receiving the material. In some cases, the application of material requires spraying of irregularly shaped contours, such as in parking lots, around safety islands, junctions joining on/off ramps and main roads or intersections. Traditionally, these operations utilized the spray bar, reset to a narrower spraying width, as the primary means to apply the material, but the finishing application was done with a hand-held spray bar.
To vary the width of the spray bar, each of the nozzles along the length of the spray bar includes a valve which can turn the nozzle on or off. In addition, the spray bar is known to include hinged outer sections which may be raised to out-cf-service positions in which flow through the nozzles of the wings is prevented. Spray bars equipped with hinged sections are particularly useful in areas where the overall length of the spray bar may physically interfere with a nearby structure, e.g., posts, walls, high curbs and the like.
When the effective length of the spray bar is changed, however, it is necessary to adjust the drive ratio of the hydrostatic transmission if it is desired to maintain the previous application rate. In the past, the selection of the new drive ratio has been a time-consuming operation requiring either fairly complex computations or another trial run. In addition, since the change in the drive ratio required manual adjustment, the risk of human error is created. Often, these manual attempts to adjust the drive ratio resulted in application rates which were noticeably different from the previous setting. Moreover, when a tapering in the road surface is encountered, it is not practical to expect an operator to successfully make the rapid adjustments required to maintain a constant application rate. Because of these inconveniences and problems, operators commonly forego the use of a shortened spray bar and, instead, use the handspray to finish all areas which cannot accommodate the full width of the bar. Obviously, this increased use of hand spraying substantially increases the post of surfacing.
More recently, mechanisms have been developed which eliminate the necessity for complex computations or trial runs when changing the width of the spray pattern. One example of such an apparatus is disclosed in U.S. Pat. No. 3,330,443 to Etnyre, which is assigned to the assignee of the present invention. The Etnyre patent utilizes an indicator arm, positioned automatically as an incident to the selection of the proper drive ratio for the hydrostatic transmission, which indicates the proper adjustment to the drive ratio if the effective width of the spray bar is changed. Although this apparatus represents a significant improvement in that complex computations and additional trial runs are eliminated, the apparatus still requires a manual adjustment of the drive ratio by the vehicle operator in response to any change in the length of the spray bar. Consequently, maintenance of a constant application rate is vulnerable to human errors made during the adjustment process.
Attempts have been made to totally eliminate the need for manual adjustments in order to hold the application rate constant after a change has been made to the effective length of the spray bar. These attempts have met with limited success. In one attempt, a pressure relief valve was placed in the routing network for the material in order to return the material to the tank when the pressure of the material in the spray bar surpassed the pressure setting of the valve and allowed material to return to the tank. Because a change in the effective length of the spray bar results in an increase in the pressure of the material flowing through the spray bar, the pressure relief valve inhibits an increased flow of material through the nozzles and, as a result, and increased application rate.
By providing a relief valve which functioned to reroute material back to the tank when excessive pressure of the material occurred, the pressure in the spray bar was regulated and, thus, a constant application rate could be maintained. Unfortunately, this approach required the operator of the vehicle to drive at a constant speed which, in practice, is not possible. Moreover, there was a second-order relationship between changes in the length of the spray bar and changes in pressure. Therefore, the relief valve functioned to maintain a substantially constant rate of application without substantial error for only small changes in the length of the spray bar. In addition, because the approach included return of extra flow when the pressure began to rise, there was no accurate way of determining the total amount of material being pumped through the nozzles. This rendered useless any attempt to determine an application rate by measuring the gallons per minute pumped by the discharge pump.
In another attempt to control the application rate, a hydrostatic transmission driving the discharge pump includes a pressure compensator which adjusts the drive ratio of the transmission so as to maintain constant pressure in the fluid lines connecting the pump and motor comprising the transmission. Unfortunately, this approach is plagued by the same problems of the previous approach which controls pressure in the return line for the spray bar, i.e., non-linearities and intolerance of variations in ground speed.